1. Field of the Invention
The present invention relates to a method of coding time-varying picture signals utilizing a bandwidth compression technique and an apparatus for implementing the same.
2. Description of the Related Art
Images are classified as still pictures or dynamic pictures. The latter varies in its statistical characteristics with the passage of time and in the following description its signal will be referred to as a time-varying picture signal. Code-transmitting techniques for the time-varying picture signals which utilize an interframe correlation are known in the art.
FIG. 1 illustrates a conventional system for coding time-varying picture signals by utilizing an interframe correlation, in which a plurality of channels, for example, four channels, are transmitted in a block. Conventionally, the transmission clock of channel A, B, C and D is each fixedly given from multiplexer 80 to each of encoders 81, 82, 83 and 84, respectively, so that the output of encoders 81 through 84 may be multiplexed by multiplexer 80 for transmission. Let us assume that the transmission rate of transmission line 85 is, for example, 40 Mbps and that 10 Mbps is allocated to each channel. In that case, since the time-varying picture signals entered to each channel differ from each other, the amount of coded information will vary for each channel or time, as shown in FIGS. 2(b), (c), (d) and (e). Although, as described above, 10 Mbps of information is allocated for each channel, it can often happen that, depending on the degree of motion of the entered time-varying picture signal or the nature of the pattern, the 10 Mbps of information is not fully utilized. Since the transmission rate of the transmission line is constant (40 Mbps), if the sum of the coded information of each channel is below 40 Mbps, then, by filling some dummy data into a margin which is left to fill up the 40 Mbps, it is controlled so that the transmission rate of the line becomes constant (40 Mbps). If the signal is decoded at the receiving side by a single decoder, then each channel of the transmitted multiplexed signal is separated by a demultiplexer, so that any arbitrary channel output is selected as the input to the decoder.
However, when the coding is carried out by utilizing the interframe correlation, if the channel selected at the receiving side is switched to another channel, then a decoding error can occur and a reproduced image can collapse, because the predicting signal of the coder at the transmitting side and that of the decoder at the receiving side do not coincide with each other.